ANODES

For any tube there is a maximum amount of power that can be dissipated safely by the anode, or plate, if reasonable tube life is to be obtained. The safe anode dissipation of transmitting most important factors controlling the amount of power the tube is one of the tube will deliver. Anodes can be classified according to the principal method of cooling employed. In some types of tubes the anodes are cooled almost entirely by radiation; in others, by conduction. Only the first type will be considered here.

In a radiation-cooled tube, the anode is operated at a fairly high temperature and heat is radiated directly by the anode to and through the walls of the bulb (generally of glass). It is usually necessary to operate such anodes at fairly high temperatures in order to keep their physical dimensions commensurate with the desired electrical characteristics of the tubes

Operation of anodes at such high temperatures brings up numerous problems. The liberation of gases from the anode itself is one of most important. In the raw state, all materials suitable for anodes contain gases -mainly hydrogen, nitrogen, carbon monoxide, and carbon dioxide -which are present throughout the body of the material. The major portion of these gases muse be driven out of the anode during the manufacture of a tube so that in subsequent normal operation no appreciable quantities of gas are liberated. The assembled tube is sealed to a vacuum system where the glass bulb can be "baked" to free it of adsorbed gases. The anodes are heated in two ways. One method is to supply a high positive voltage to the anode and bombard it with electrons from the cathode. Another method is to place around the glass bulb a coil carrying high-frequency currents. The anode then acts as a short-circuited secondary of a transformer and is heated to a high temperature by induced currents.

Some of the most important considerations in the choice of an anode material for radiation·cooled tubes are its thermal emissivity, its mechanical properties, and its vapor pressure.

The thermal emissivity should approach as nearly as possible the ideal of a black body, in order to obtain the highest dissipation rating for a given anode design and anode operating temperature (the temperature being determined by gas liberation). At thought it might appear that the size of the anode could he increased to get the desired dissipation raring for an anode of a given material. However, this usually results in an increase in the electrostatic capacitance between the anode and the other tube electrodes: it also increases the weight of the anode which means heavier mounting supports and a larger mass of material from which gases must be removed. Because of the pronounced trend to higher frequencies in radio communication, it is important to keep interelectrode capacitances to a minimum so that capacitance charging currents which entail losses, can be limited to reasonable values.

The mechanical properties of an anode material are very important. The material must be capable of being worked readily into the desired shapes and must maintain these shapes at the high temperatures employed during tube manufacture. Only a very small amount of warping an be tolerated at the normal operating temperature because warping may produce a change in electrical characteristics of a tube.

The vapor pressure of an anode material must be low enough so as not to cause appreciable metallic deposits in a tube during manufacture. Such deposits on the insulators in a tube may result in excessive interelectrode leakage or in excessive radio-frequency losses in the insulators.

Various materials have been used for transmitting-tube anodes. A brief description of the materials which have been most widely used is given in the following paragraphs.